It is known, as disclosed in U.S. Pat. No. 4,161,219 that the effects of hydrostatic forces in the hydraulic control line acting on a subsurface well safety valve can be reduced by using a small area, rod type piston and cylinder assembly. This allows the safety valve to be set at greater depths and/or reduce the required biasing force. Presently conventional rod piston and cylinder assemblies use a quarter inch diameter rod which provides a hydraulic working area of 0.049 square inches. It does not appear to be feasible to provide much smaller areas. Therefore further reducing the hydraulic area in order to overcome hydrostatic forces appears to be limited. However, the biasing means, generally a power spring in the subsurface well safety valve, is used to move the valve to a closed position. The biasing spring must not only overcome the hydrostatic force acting upon the piston and cylinder assembly, but must move the flow tube to the closed position. Therefore, the biasing power spring must be sized and uses a large percentage of its power output, particularly when using small area hydraulic area piston and cylinder assemblies, to overcome the weight of the flow tube.
One feature of the present invention is the provision of providing a light weight, non-ferrous flow tube which reduces the biasing force required out of the spring and/or allows the valve to be set at greater depths.
Another problem in safety valves is that the spring which moves the flapper to the closed position require restricted design parameters. Another feature is the provision of a lightweight, non-ferrous flapper which again allows for the use of a smaller, weaker and less expensive spring.